The conventional method of reverberation chamber testing uses a large metallic paddle wheel to mix or stir the fields within the chamber. This ensures that the test object will be exposed to the maximum field values in the chamber. The frequencies at which eigenmodes can exist in a rectangular cavity of dimensions a, b, and d are: ##EQU1## where: C=the speed of light
m, n and 1 are integers PA1 u.sub.r =relative permeability PA1 C.sub.r =relative permeability PA1 U.S. Pat. No. 4,941,134, issued to Nyberg et al; PA1 U.S. Pat. No. 3,720,402, issued to Cummins et al; PA1 U.S. Pat. No. 3,614,069, issued to Murry; PA1 U.S. Pat. No. 3,495,807, issued to Shields et al; and PA1 U.S. Pat. No. 2,919,215, issued to Neuhauso
There has been an increasing interest in the use of mode tuned or stirred reverberation chambers for conducting electromagnetic coupling and upset experiments. Mode stir-chambers consist of a high quality factor metallic reverberation chamber in which a stirring device such as a metallic paddle wheel is used to incrementally or continuously alter the boundary conditions within the chamber. The goal is to achieve an isotopic homogeneous field everywhere within the volume of the cavity except near the walls. Field homogeneity is achieved in this manner by exploiting the pseudo-statistical nature of each of the eigenfunction's contribution to the field level at a given point within the chamber volume. Extensive work has been done at the National Institute of Standards and Technology (NIST) and at the Naval Surface Weapons Center to both optimize chamber design and characterize chamber fields. Field uniformities of less than +3 dB above 1 GHz and less that +2 dB above 2 GHz have been achieved in a 2.74.times.3/05.times.4.75 m welded steel enclosure using the paddle wheel technique.
Reverberation chamber testing offers several advantages over anechoic or plane wave illumination testing. For example, E-field levels in the thousands of volts per meter can be achieved using relatively low power sources such as 200 watt TWT amplifiers. Coupling cross sections can be measured independent of the angle of incidence, and this is particularly beneficial when testing subsystems that would normally exist inside an equipment bay or cavity. On the other hand, dependence on mechanical mode stirring can complicate data acquisition and interpretation as well as require long periods of time (sometimes as much as ten hours per test) to acquire a complete data set. These complication are primarily due to the requirement that the data be measured for many differ positions of the paddle wheel for each frequency and power level because field uniformity inside the volume of the chamber can only be obtained by averaging the fields influenced by each position of the tuner over many positions. Hence, real time field uniformity can only exist on time scales of the same order as the paddle wheel rotation rate.
This invention uses electronic mode stirring in a reverberation chamber to perform both coupling and upset testing. While the conventional method of mode stirring holds the frequency constant and varies the cavity's boundary conditions in order to obtain a sufficiently large sample of eigenfunction contributions to the field levels at a given point in the chamber, the method described here does not vary boundary conditions, but instead averages the eigenfuctions's contributions over a narrow band of frequency.
In mechanical mode stirring, these resonant frequencies are perturbed or stirred by shape perturbations. The degree to which the shape of the cavity can be perturbed is related to the size and number of paddle wheels as well as other physical characteristics such as the relative angle of the paddle vanes. The advantages of this technique over the conventional paddle wheel system and method include shorter test times, simplified data acquisition and control, and more interpretable results. The invention was reduced to practice with good results, once using a reverberation chamber with dimensions of 6.times.4.times.5 feet, and in a second chamber with dimensions of 3.times.3.times.3 feet.